While the NBC miniseries Asteroid dramatized the threat asteroid impacts are to the Earth, satellites in orbit around the Earth face a much more mundane -- but equally dangerous -- threat: orbital debris, or space junk.
     During last month's repair of the Hubble Space Telescope, astronauts found a small hole in the dish of one of the telescope's antennas. A collision with a small piece of space junk is considered the likely cause. During the repair mission the shuttle, with the Hubble docked in its payload bay, had to fire its maneuvering jets to avoid coming "dangerously" close to a fragment from a Pegasus rocket which exploded several years ago.
     Not all spacecraft are as lucky. Last year ground controllers lost contact with CERISE, a small British-built satellite. Weeks later, engineers used the data from the satellite to discover that the satellite has been struck with a fragment from an Ariane rocket which had exploded several years ago. The collision broke off part of the satellite which helped maintain its stability.
     Orbital debris is becoming a major concern for those planning spacecraft missions in Earth orbit, either in low Earth orbit (LEO) or in geosynchronous orbit (GEO). It is a concern that will grow as constellations of dozens and hundreds of communications satellites are launched into LEO, such as Iridium and Teledesic, and while large targets for space junk are constructed, namely, the International Space Station.

Counting the Debris
The problem with counting orbital debris is that even tiny pieces of junk can cause significant damage. At orbital velocities of over 28,000 kmph (17,500 mph), an object as small as 1 cm in diameter has enough kinetic energy to disable an average-sized spacecraft. Even objects as small as 1 mm can damage sensitive portions of spacecraft.
     A 1995 NASA study, "Guidelines and Assessment Procedures for Limiting Orbital Debris", notes that there are about 6,000 objects in LEO being tracked by the U. S. Space Command. Of these only about 5 percent are operating spacecraft. More than half are fragments from on-orbit explosions, like the Ariane explosion which created the debris that disabled CERISE. About 40 percent are dead spacecraft and upper stages of rockets left in orbit.
     However, that 6,000 is only the tip of the iceberg. It represents only those objects that can be tracked by groundbased radars. The 1995 NASA study estimates that there are 15,000 objects in LEO 10 cm or larger; one of those is enough to "catastrophically fragment" a spacecraft, adding to the debris problem.
     The same study estimates that there are about 150,000 fragments 1 cm or larger, enough to disable a spacecraft, and more than a million objects 1 mm or larger. Finally, the study estimates that there is over a billion tiny particles as small as 0.1 mm in diameter -- mostly remnants from solid rocket motor firings -- that still have enough energy to wear down spacecraft surfaces.
     Not all the news about orbital debris is bad. In 1996 NASA updated its models for estimating the amount of orbital debris in LEO. The new model incorporated radar observations from Haystack Observatory in Massachusetts with a refined dynamical model. The new results found that the population of objects 1 cm or larger in orbit might be just half the number estimated by a model five years before.
     Nonetheless, even if there are only 75,000 objects 1 cm or larger in LEO, it's cause for concern for spacecraft planners. Debris this size is large enough to disable a spacecraft, but too small to be tracked and avoided. The only way to deal with this debris is to add as much shielding as feasible, and hope the spacecraft is lucky through its planned mission.
     The 40 percent of the trackable objects in LEO that are old spacecraft or upper stages can make the debris problem even worse by becoming sources of even more small debris. This can happen either when the object is stuck by current space debris, fragmenting the vehicle into hundreds or thousands of smaller pieces. This can also happen of the spacecraft's own accord, through such events as exploding batteries or bursting fuel tanks.

Space Junk and the Space Station
Orbital debris has become a concern for those planning even small spacecraft missions into LEO. It's become a major concern for those working on what will become the largest structure in LEO, and thus the largest target for debris: the International Space Station.
     The threat of impacts by orbital debris has been a concern throughout the design of the space station. On a list of top ten hazards for the space station compiled last year by the NASA Safety Office, the threat of impacts by micrometeorites and orbital debris ranked sixth ("extremely hazardous EVAs" to repair space station solar panels was on top of the list.)
     In January, a National Research Council committee released a study evaluating NASA's efforts to study the threats of orbital debris. While largely positive, the committee did find some areas of concern about plans to protect the station from debris.
     "Overall efforts to protect the space station have been extensive and thorough," said committee chair George Gleghorn, a retired vice president and chief of engineering at the TRW Space and Technology Group.
     "However," he noted, "the space station will be particularly vulnerable to collisions due to its size and because it will be in orbit for at least 15 years."
     The committee's greatest concern was about how well Russian-built modules would be able to deal with the impact threat of space junk. The committee thought the Russian contribution to the station was not sufficiently shielded to protect the station's occupants.
     The ability of a particular module to withstand an impact from orbital debris is measured using a computer module. Given the known size and shielding of a module, and the estimated population of orbital debris, engineers calculate the "probability of no penetration" (PNP), that is, the probability that a module will not be penetrated by debris in a ten-year period.
     The overall goal of the space station is a PNP of 0.81, which is a 19 percent chance that the station will be penetrated by orbital debris at least once in its first ten years. This is mandated by requiring the Russian contributions have an overall PNP of 0.90, and the American and other modules have an identical overall PNP.
     While the American and other modules have had little problem reaching their goal, the Russian modules have fallen far short of their goal of a 0.9 PNP. A design review in 1994 found the Russian sections to have an abysmal PNP of 0.12, making it almost certain they would be penetrated at least once by orbital debris.
     Since then the Russians have improved the quality of their shielding, but their PNP is still only 0.6, bringing the overall probability of no penetration for the station down to 0.55 -- far short of the goal of 0.81.
     Although the new orbital debris models improved the PNP to 0.85, above the goal, the committee still expressed concern about the lack of shielding in the Russian-built Service Module. At the time of their report, plans were to add additional shielding to the module after launch in 1998. Since then, the launch of the module has been delayed, likely to no sooner than the end of 1998 and possibly later. There has been no indication if this delay will allow engineers to add additional shielding to the station before launch.
     Shielding for the station generally involves a system called a "Whipple bumper." A thin outer layer of material, usually aluminum, is placed a short distance in front of the module wall. The aluminum bumper is designed to vaporize any junk that hits it, and spreads any remaining debris out into a larger area that can be absorbed by the module wall, or "catcher" below the bumper.
     The shielding is designed to absorb impacts of objects up to 1 cm in diameter. Beyond that, it can only minimize the amount of penetration the debris creates in the station module. Unfortunately, objects larger than 1 cm up to 10-20 cm cannot be tracked and avoided reliably, leaving a window of vulnerability to the station from medium-sized debris. Until shielding or tracking can be improved, there is little to do but hope that the station is lucky.

Dealing with the Problem
There have been a number of schemes developed to counter the problem of orbital debris. Proposals have ranged from garbage-collecting spacecraft to lasers which vaporize parts of debris, creating an impulse which pushes them into orbits that speed their demise by burning up in the Earth's atmosphere. However, none of these proposals will likely be implemented any time in the near future, for reasons of cost if nothing else.
     The emphasis of NASA's plans for dealing with space junk has been prevention. The 1995 NASA study which looked at orbital debris listed several guidelines for minimizing the problem. It suggested dying spacecraft and booster stages be pushed into "graveyard" orbits outside of LEO and GEO, where even if they break apart, they will contribute little to the orbital debris in the more populated orbits.
     The study also suggested steps be taken to prevent the creation of debris from spacecraft and booster stage explosions by venting fuel which might burst fuel tanks and discharging batteries which might explode.
     Similar sentiments were expressed by those attending a recent international forum on the problem of orbital debris, organized by the United Nations in Vienna. These was considerable interest with dealing with space junk in GEO, an increasingly-crowded orbit that has its share of defunct spacecraft and space junk.
     One proposal was to require spacecraft to move 300 km (180 mi.) beyond GEO when they have reached the end of their lifetimes, to make room for new spacecraft and to limit the possibility of the defunct spacecraft contributing to the debris problem.
     The Vienna meeting is part of a larger 5-year plan by the UN to put together guidelines to minimize the creation of space junk.
     Until we can find the money for debris-deflecting lasers or garbage-collecting spacecraft, it appears the key to reducing orbital debris is the same lesson taught time and time again on Earth: don't litter.